Due to transmit power limitations in mobile user equipments, the need for higher through-puts in future telecommunication networks, especially near the cell edge, combined with the constraint on the uplink link budget will necessitate the introduction of smaller cell sizes compared to cell sizes typically deployed in present cellular systems. The use of smaller cell sizes (pico cells) can be deployed in different carrier frequencies or can be overlaid in the same carrier frequency as the larger cells (macro cells). New interference scenarios appear with heterogeneous deployments in which small cells, the pico cells, and large cells, the macro cells, may use the same carrier frequency. This is primarily due to the large imbalance between the transmit (Tx) power of macro and pico base stations (BSs) and the applied cell association method that defines the base station that is responsible for a particular user i.e. serving base station serving a particular user equipment.
A known cell association mechanism is based on Reference Signal Received Power (RSRP) measurement from the user equipment (UE). RSRP is dependent on the transmit power of the base station. In the case of heterogeneous deployment with macro and pico cells as discussed above, the RSRP-based cell association leads to suboptimal performance in the uplink (UL). A user equipment may measure a higher RSRP from the macro base station although it is located closer to the pico base station, i.e. its pathloss to the pico base station is smaller than the pathloss to the macro base station. In an uplink, a cell association based on pathloss may be used.
If the cell association is modified to extend the area where the pico base station is a serving base station, a new interference scenario exists in the downlink (DL) direction. Macro base stations keep their transmit power and thus cause strong downlink interference to user equipments served by a pico base station but located close to the RSRP coverage area.
Similar or opposite interference problems exist in other configurations of heterogeneous deployment. User equipment served by a macro base station may also be strongly influenced by a pico base station in the downlink, if it is close to the pico base station but its access is restricted to a group of subscribed users.
One way to reduce downlink interference perceived by a user equipment is to apply codebook-based coordinated beam forming. In multiple antenna systems base stations weight their downlink signal with a precoding matrix before transmission. With coordinated beam forming, a base station uses a certain precoding matrix for which the received signal strength at a certain user equipment (in an adjacent cell) is low. In codebook-based coordinated beam forming a base station selects from a predefined codebook the precoding matrix for which the interference level perceived at a certain user is minimum.
In coordinated beam forming, the precoding matrix computation at a base station requires certain knowledge of the channel from the base station to the user to protect from interference. In time division duplex (TDD) systems, explicit downlink channel knowledge at base stations can be assumed due to channel reciprocity of the uplink and downlink channel.
In frequency division duplex (FDD) systems, however, base stations cannot estimate downlink channel coefficients alone. Since feedback from user equipments is needed to acquire channel knowledge at base stations in FDD systems, implicit downlink channel knowledge at the base station is the more practicable solution to apply coordinated beam forming. In this case a codebook of precoding matrices is defined as known from 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation. Each precoding matrix is addressable with its index within the predefined codebook. User equipments only report the index of the precoding matrix which is more suitable for a certain purpose, e.g. for maximizing or minimizing the received signal strength.
The problem with existing codebook-based coordinated beam forming for FDD systems is the need for additional user equipment feedback. This UE feedback, however, complicates the beam forming and leads to additional signaling.